Composition and method for cleaning aluminum preparatory to spot welding



Patented July 26, 1949 COMPOSITION AND METHOD FOR CLEANING ALUMINUM PREPARATORY TO SPOT WELDING Emmette R. Holman, Pasadena, Calif., assignor to Turco Products, Inc., Los Angeles, Calii'., a corporation of California No Drawing. Application July .6, 1942, Serial No. 449,904 Y 9 Claims. 1 This invention has to do with the finishing of aluminum surfaces.

To prepare aluminum surfaces and the surfaces .of aluminum alloys for welding or painting,

and particularly for spot welding, it is desirable 5 treatment. 7

first to finish the surface by removing the oxide I attribute those shortcomings to the unconand otherwise thoroughly cleaning it, which opertrolled, highly corrosive solutions heretofore emation will be hereinafter characterized as' deoxiployed; to the failure to take into account the dizing. activating or catalyzing agents present as im- This invention is directed more particularly to 10 purities in the chemicals and tap waters used; to an aluminum cleaning bath which may be used the use of heat as the accelerating agent and to coldthat is, at or around ordinary room temperthe difficulties of maintaining any dependable atures ranging typically from about 65 F. to control of the pH value-and the activation of such 85 F. baths.

The aluminum deoxidizing baths of the prior It is among the chief aims of my invention to art have been of both the cold and hot'type, provide an aluminum deoxidizing bathwhich is but all have been subject to rather serious shortfully controlled in its attack upon the surface comings. being treated, and in which overor under-treat- The cold baths of the prior art have been ing may be avoided. necessarily highly corrosive to be operable, de- It is a further objec to provide an effective pending for their effectiveness upon a relatively and economical maintenance of such a bath at a rapid attack upon the surface being treated-a uniform p Value to mp a for the usual typical immersion period being of the order of 30 depletion of the acidity caused by the neutralizseconds. Moreover, they are somewhat selective ing effect of the metal which it dissolves. in their attack, with the result that they 'com- It is also an object to provide a bath which pletely dissolve only certain surface constituents Will operate at ordinary room temperatures, withand leave the undissolved and galvano-deposited out the necessity of heating or cooling means. constituents on the surface of the work in the Another object is to reduce the number of steps form of a smut which requires a subsequent rerequired in the processing cycle by' eliminating the moval operation. final bright dip or wiping operation as well as a The heated bathsthat is, baths which, for corresponding number of rinses. activating purposes, are necessarily heated to Another object is to minimize the deposition on temperatures ranging from 120 F. to 212 F. the work of corrosion promoting elements which have been subject to substantially the same obtend later to cause failure of the work by atmosjections, plus the impossibility of maintaining 5 pheric or electrolytic corrosion. j uniform temperatures under practical working It is another object of my invention to p' ovide conditions, which latter is a major objection since a bath of this character which may be ed in those prior art baths require a rather critical inexpensive equipment. time-temperature relationship. For instance, ar- Another object is the provision of an al I inum ticles having relatively heavy or thick sections 40 cleaning bath in which there is an absence of are slower in coming up to the temperature of hazardous fumes and which therefore requires no the bath than are articles having lighter sections, ventilating equipment. with the result that, the two are attacked by the Still further advantages are inherent in my 7 bath at different speeds. And when heavily loadinvention and how those as well as the foregoing ed baskets of work are immersed in the bath, they specified objects are attained will be best underchill the bath long enough to upset the delicate stood from the following detailed explanation of time-temperature relationship of the method em.- a preferred adaptation thereof. 7 ployed. I In general, I utilize a cold bath which is rela- Some of the practical difficulties experienced tively slow inits' attack upon the surface being with prior art practices may be'better understood treated, which pnovides a very low pH value, and

when we consider the fact that not infrequently, due to the rapidity of the attack of those baths, it is found difiicult to get a large sheet into and out of the bath rapidly enough to prevent overwhich may be economically maintained at its original low pH value. Preferably I employ a solution having a phosphoric acid base augmented by the addition of a highly dissociated, nonhalogen oxygen mineral acid in sufiicient quantity to bring the pH value of the bath down to the desired low figure, the bath being maintained at the desired value by subsequently adding, from time to time, additional quantities of the latter acid. Moreover, I utilize a chemical agent as the catalyzer or activator, after firsttaking into account the activating radicals present as impurities in the acids and water of the bath, so that at all times I am able to maintain accurate control and uniformity of operation.

I am aware that phosphoric acid has been used as an oxide removing agent for aluminum, but heretofore it has been deemed; necessary to use it in relatively strong concentrations, of the order of by weight or more, in order to .obtain a bath of operable acidity. It has also been deemed necessary, in prior uses of phosphoric acid baths, to maintain the bath at relatively high temperatures ranging from 120? F. to 212 F. in order to activate the bath in its attack upon the work. o a owan e hasbe nmade. n. th p r a t practices for the. activating radicals present as impurities in commerciahgrade acids and in tap waters, and, this failure, added to the difliculties of maintaining a uniform bath temperature, have resulted 1 in, frequentinjurioutsover-treatment of the surfaces, particularly in, the case of relatively thin sheets,

The. previous, necessity, for. using relatively strong solutions.-oi-piiosnhqric-acid I attribute to the tti basic nature of the chemical. That is, ortho-phosphoric acid; comprises three available acid hydrogen. ions; of? widely variant acidity. Only one of those ions is really strongly acidic. Consequently, it has been found-that only very strong solutiensoiwthe. phosphoric acid will provide the high acidityeor. hydrogen ion concentratime necessary to,,disso,l ve aluminumoxide and to prevent the formation of undesirable deposits of aluminum. phosphate on the surface.

In the, preferred adaptation of my invention, I use, as the baseo nmy etchingsolution, a relatively weakconcentration of..ortho.-,. pyro-, meta, or polyphosph,oric acid; the, optimum being of the order of, about,5%.;by weight, I obtain the desired acidity of the bath by augmenting it with a relatively inexpensive, highly dissociated, nonhalogen oxygen mineral. acidspreferably sulphuric acid,- although, nitniconchromic acid will sufiice. I add such; additional minute'quantities of this dissociated acid; asthe acidityof 'the bath becomes depleted, as may be necessary to maintain the I initial pH value;

Ipreier initially to prepare, and to maintain the bath at a pH value, of;below;1.0 the optimum being about 0.5, although I do not limit my intention to such figures since I find-mybath to function at pH values ranging from below'zero upwardly toabout three, T br-ing my bath to a 0.5 pH value the quantityoiithe;augmenting acid required depends upon; the alkalinityof the tap water used. To a typical watercarrying a total alkalinity- 0f, 300; parts ;per mil lion expressed as CaCOg, I add 4%;by weight of concentrated sulphuric acid (specific. gravity 1.3.4). 7

I have said that one ottheoutstanding shortcomings 0f P .as vs fac ices asbeen the a sence of any uniiorm actiyationpr the attackof e. bath.landfi ls-stimulant e nes e In ll au tsr e wcqmmseq a aerase' em a there are present, as impurities, radicals which act as catalyzing agents. No account has heretofore been taken of those radicals, with the result that as the solutions have been heated to activate etching activity, they have been somewhat erratic in their attack, resulting in overtreatment and injury to the Work. Furthermore, apart from the presence of such impurities, as hereinbefore stated, reliance upon activation by heat has been unsatisfactory for the-reason that, under practical working conditions, it is diilicult to maintain a uniform temperature of the bath and it also requires relatively expensive equipment and produces hazardous fumes.

I find that the impurities most responsible for this activating efiect are the halide radicals, particularly the chlorides and fluorides, and in my bath I positivelyqand accurately control the 7 action by-iirst determining the amount of active radicals present in the bath. If there is an excess over and aboveenough to cause the activation desired to provide a given immersion period, I modify or neutralize them to the necessary extent by adding well-knoyvn neutralizing chemicals such, for instance, as silver nitrate for the chlorides and iodides; or. calcium.nitrate for the fluorides, or if there is an insufficient amountof them in the bath. Iaddl a: suitable activating agent in an .amount'sufii'cient ,tolprovi'de the-desired etching rate. Preferably; I addl'sodium fluoride, NaF, becauseit is. an .eflicientcatalyst, it is non-hygroscopic, non-corrosive: and maybe safely handled with the bare. hand: Furthermore, it is in finely divided powder form suitable for measuring in rninute; quantities.: Other. halides are usable, however, amongthemlbeing chlorides, iodides, bromides, bifluorlde. of? sodium' and the fluorides and bifiuorides: oi otlier metals: such as potassium, aluminum, lithium and magnesium. Also the si1ic0fiu0rides,: fluoborates :and alu.minofluorides may. be.-'used":since they: dissociate to some extent. into free hydrofluoric acid under the strong acid conditions of the bath.

The total'concentration of the catalytic agent which I find preferable under normal conditions is from .01% to 2.0'% by:Weight;;the principa1 determining COIISidETatiOIISnbBiIIgthe immersion time assigned .to theoperation. The. desired immersion period .is generally: oiithe orderofilfour minutes, although it may vary from .30 seconds to 20 minutesindiiierent plants. For amour minute immersion period-I z'prefer: to useabout 0.2% by weight ofcatalyst; Also I'might mention that the gage: oi thealuminum. sheet. being treated may be a contributingfactorin determining the optimum catalyst content at the bath as well as the immersiontime; because. lighter gage sheets, or thosewhosesuriaceshave been severely worked mechanically, .andcheavier gage sheets may respond; to the: bath: differently. During thecourse of. immersing-in. and: removing work from, the bath; somet-of: thecatalyst. is usually taken. out of the=batlr on:- thework, and. my invention contemplates;-subsequent.' addition; of suificient. cata y o compensate-for. suchi de ict tion.

Inthe operationof; my :bath; the rate. of. attack on the metal: is relatively-.slowzand; consequently the qu n ity of. irritatingriogrin 'the,. surrounding atmosphereis materiallyareduceds There are no hazardousfumes; I amable topliminatetheiog entirely byintroducing.intothe batha synthetic detergent and foaming agent, commonly .known as a wettingsagent... Suitablelwetting agentsare the c dstable ;sulpbonatedxocrsu phat -t1 i 'w rivative's of long chain hydrocarbons, alcohols,

1 fatty acids or substituted aromatic compounds, or they may be phosphonated derivatives of, those compounds,.or substituted nitrogen compounds 'havingsurface active properties, or they may. be

glucocides'whichexhibit foaming properties. I prefer to use the sulphonated phenolic ethers, or the alkyl aryl sulphonates. 'The wetting agent provides a blanket of foam which collects the hydrogen gas bubbles and retains the entrained liquidwhich would otherwise be'carried into the maximum bubble size so that the hydrogen gas bubbles will detach themselves more readily from the aluminum surface being treated and thereby permit eiiicient penetration of the bath.

3. A method for cleaning. the surface of aluminum articleswhich comprises: immersing sucradical, withdrawing the'cleaned' articles from the bath in about 20 minutes" to'about 30 seconds, said time varying inversely with the concentration of said substance, adding successive increments ofanacid to maintain the acidity The efficiency of my bath has been particu- I larly demonstrated in its application to finishing aluminum surfaces preparatory to spot welding operations. For instance, the prior art aluminum deoxidizing baths are so inefficient that, in the aircraft industry for instance, spot welding is allowed only on secondary structures, and even then production is necessarily slowed for frequent cleaning of electrode tips. Present baths and methods permit only about 100 spot welds between cleanings of the tips. On the other hand, by my bath and process, the surfaces are so efiectively finished and smut-free that welding of primary structures is possible and more than 400 spot welds can be made between cleanings of the electrode tips.

While I have named specific chemicals and procedures in course of explaining a presently preferred adaptation of my invention, I wish it understood that, within the broader concept of my invention as defined by the following claims, it contemplates the use of equivalent chemicals, steps and procedures.

I claim:

1. A method for cleaning the surface of aluminum articles which comprises immersing successive aluminum articles in a cold aqueous phosphoric acid bath having a pH below about 3 and an initial concentration of about 0.01% to 2.0% w. of a substance capable of yielding a halide radical, withdrawing the cleaned articles from the bath in about 20 minutes to about 30 seconds varying inversely with the concentration of said substance, and adding sufficient of said substance to the bath from time to time so as to maintain its concentration approximately within this range.

2. A method for cleaning the surface of aluminum articles which comprises: immersing successive aluminum articles in a cold aqueous bath, which bath has a pH below about 3 and comprises a phosphoric acid, sulfuric acid and an initial concentration of about 0.01% to about 2.0% w. of a substance capable of yielding a halide radical, withdrawing the cleaned articles from the bath in about 20 minutes to about seconds, said time varying inversely with the concentration of said substance, adding successive increments of an acid to maintain the acidity of the bath, and adding successive increments of said substance to maintain its concentration in the bath.

of the bath, and adding successive increments of said substance to maintain its concentration in the bath.

4. A method for cleaning the surface of aluminum articles which comprises: immersing successive aluminum articles in a cold aqueous bath, which bath has a pH below about 3 and comprises a phosphoric acid, a highly dissociated non-halogen mineral acid and an initial concentration of about 0.01% to about 2.0% w. of a substance capable of yielding a halide radical, said substance comprising a fluoride, withdrawing the cleaned articles from the bath in about 20 minutes to about 30 seconds, said time varying inversely with the concentration of said substance, adding successive increments of an acid to maintain the acidity of the bath, and adding successive increments of said substance to maintain its concentration in the bath.

5. A method for cleaning the surface of aluminum articles which comprises: immersing successive aluminum articles in a cold aqueous bath, which bath has a pH below about 3 and comprises a phosphoric acid, a highly dissociated nonhalogen mineral acid and an initial concentration of about 0.01% to 2.0% w. of a substance capable of yielding a halide radical, said substance comprising a silicofluol'ide, withdrawing the cleaned articles from the bath in about 20 minutes to about 30 seconds, said time varying inversely with the concentration of said substance, adding successive increments of an acid to maintain the acidity of the bath, and adding successive increments of said substance to maintain its concentration in the bath.

6. A phosphoric acid cleaning bath having a pH between about 0 and about 3 and comprising about 0.01% to about 2.0% w. of a substance which furnishes a halide radical.

7. An aqueous cleaning bath having a pH between about 0 and about 3, said bath comprising: a phosphoric acid, a highly dissociated nonhalogen mineral acid and about 0.01% to 2.0% w. of a substance which furnishes a halide radical.

8. An aqueous cleaning bath having a pI-I between about 0 and about 3, said bath comprising: a phosphoric acid, sulphuric acid and about 0.01% to about 0.2% w. of a substance which furnishes a fluoride radical.

9. A method for cleaning the surface of aluminum articles which comprises: immersing successive aluminum articles in a cold aqueous bath, which bath has a pH below about 3 and comprises a phosphoric acid, a highly dissociated non-halogen mineral acid and an initial concentration of about 0.01% to about 2.0% w. of a substance capable of yielding a halide radical,

said substance comprising a fiuoborate, withdrawing the cleaned articles from the bath in about 20 minutes to about 30 seconds, said time varying inversely with the concentration of said substance, adding successive increments of an Th9 following references are 9f gecord in the file-70f this patent: .7 .1 I

UN D T TES ATENTS Number Num m Pails: 272,893 2 971 0 Ed ar -1 9 9? Great Britain Gr at Br a n 7 #8 acid to maintain the acidity of {she bath, and Number 1 Name Date "adding successive "increments of said substance 2227945 I NeilsOnfi--- Jan: 7,1941 330' maintain its concentration in the bath? 2,256,449 George Sept. 16, 1941 EMMETTE R.-HOL1\[AN. 2,257,960 Humphrey; Oct.1-'T, 1941 V i 2,316,219 Bro'wn et al.'; Apr. 13, 1943 REFERENCES .CITED 2,337,062 Page Dec. 21,1953

' FOREIGN PATENTS .Couritr? .Daie

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@s 'pt a 1191 

